RE: OSB is another good option as it does not delaminate like ply. It may not delaminate, but it swells & looses strength when moisture gets in. It is not stable enough to lay hard wood or tile over it (- according to the manufacturers).
As for coating the new wood, the wooden boat restorers use a 'Clear Penetrating Epoxy Sealer' to enhance the life of the wood. I read about this stuff in Don Danenberg's 'How to Restore Your Wooden Runabout'. It has been out since 1972 & has a proven history of almost doubling the life of the paint job when it is used.
If it's good enough for boats that are worth more than a new bus . . . .
The top coat of rubberized undercoating is a GREAT idea!
I agree that OSB isn't suitable.
I have used the epoxy sealer stuff (SP Eposeal 300 is the brand I've used), and although it's very good it seems very expensive for what it is - I may be wrong, but it just seems to be regular epoxy thinned down with lots of solvent. If I went that route again I would look into making my own.
There is a very good single pack product called 'Universal Clear Primer', which I have known boatbuilders to use before applying any other kind or coating material (or even bonding material, which always seems a bit odd to me). UCP gives off some very interesting fumes, so use it in a well ventilated area!